1. Field of the Invention
The present invention relates to an ophthalmologic photographic device for use in ophthalmologic clinics and elsewhere.
2. Related Background Art
The gain of an electronic image pickup element can be regulated mainly by one of the following two methods. By a first method the gain of the image pickup element can be selectively varied as desired, but the gain is fixed, instead of varying with changes in the state of exposure to light, while a second is a so-called auto-gain control (AGC) method, by which the gain automatically varies with changes in the state of exposure to light in which the image pickup element operates.
In electronic photography of ophthalmologic fluorescence contrast imaging in which the brightness of the object varies over time, a gain regulating method like AGC is more suitable where the image is continuously watched or a moving image is to be photographed under continuous illumination with an incandescent lamp or the like.
However, where a very fine image of a higher S/N ratio, i.e. an image free from the superposition of noise, is to be obtained, it is necessary to set the gain of the electronic image pickup element as low as practicable. In such a case, a continuously lit light source which is generally used for ophthalmologic equipment cannot provide sufficient light intensity for illumination, and therefore usually still pictures are taken under strobe lighting using a xenon tube or the like. However, since the exposure time is extremely short in still photography, AGC would lag behind, and no proper gain regulation could be achieved. Therefore, it is necessary to pick up an image with a preset fixed gain.
In an alignment procedure in which the fluorescent image itself need not be picked up, the light coming incident on the image pickup element may be significantly varied by such an external disturbing factor as harmful reflected light from the object eye, and accordingly a stable image is more likely to be obtained by shooting with a fixed gain.
Thus, to obtain a better image, AGC and regulation with a fixed gain are selectively used according to the conditions of use and the shooting conditions.
However, according to the prior art described above, there may be a time lag until regulation by AGC arrives at the right gain where the incident light intensity greatly varies. Moreover, as the gain at the start of AGC is usually fixed to a prescribed level, an image of proper exposure is sometimes impossible to be obtained immediately after a shift to AGC.